The present invention is related to Micro Electro Mechanical Systems (xe2x80x9cMEMSxe2x80x9d). This field includes micro-fluidics, micro-pumping, micro-valves, precise fluid dispensing, micro-molding, and micro-magnetic actuators. More particularly, the invention relates to devices and methods for microscopic pumping, mixing, dispensing, and valving of blood and reagents.
Micro-pumping for transporting small volumes of fluids is made possible by micro-pumps which use piezoelectric, surface wave, thermal, fluidic or static electric actuation to move diaphragms, membranes, flappers, wheels, or actual fluids. This has been discussed in the literature.
The term xe2x80x9cmicroxe2x80x9d refers to objects of small scale having dimensions on the microscopic level. Such a microscopic level includes orders of magnitude of 1xc3x9710xe2x88x923 meters to about 1xc3x9710xe2x88x927 meters, where 1xc3x9710xe2x88x926 meters is commonly referred to as a micron. Such orders of magnitude correspond to dimensions such as volume and mass.
Blood flow sensors have been constructed out of polysilicon rotors. The goal of such devices is to position them in blood vessels and measure the flow that might change as a result of an occlusion. Rapoport, S. D., et al., Fabrication and testing of a microdynamic rotor for blood flow measurements, Journal of Micromechanical Microengineering 1, (1991), pp. 60-65. This article is incorporated by reference herein. A rotor 300 microns in diameter was machined out of polysilicon. A two micron thick hub was attached to the center of the rotor to allow the rotor to rotate in a seven micron gap. The rate of rotation of the rotor is measured using a microscopic permanent magnet to modulate the resistance of a permalloy placed near the rotor. The change in resistivity provides an electrical signal, the frequency of which is proportional to the rotation rate, and hence the velocity.
The durability and robustness of micro rotors has been improved by adding polysilicon bearings to the point of rotation to overcome the lack of ball bearings and lubricants which exist in conventional sized rotors. Tavrow, Lee S., Operational characteristics of microfabricated electric motors, Sensors and Actuators 35 (1992), pp. 33-44. This article is incorporated by reference herein. The life of the rotor is increased significantly with the addition of such bearings.
In conventional magnetic actuators, most of the magnetic energy is stored in the gap due to the relatively large reluctance of the gap compared to the negligible reluctance of the magnetic core. However, in magnetic micro-actuators the fabrication limitation on the achievable cross sectional area of the magnetic core as well as the finite core permeability increase the core reluctance to the point that this assumption may no longer be valid. Nami, Z. et al., An energy-based design criterion for magnetic microactuators, Journal of Micromechanical Microengineering 6, (1996), pp. 337-344. This article is incorporated by reference herein. The reluctance problem is overcome by sizing the gap between the core and coils according to the actuator requirements so that the reluctance of the gap and the reluctance of the core are equal.
In order to produce a magnetic force (or actuation) at a specific location, magnetic micro-actuators should have an inductive component to generate magnetic flux to the point where actuation takes place. Ahn, Chong H., et al., A fully integrated surface micromachined magnetic microactuator with multi-meander magnetic core, Journal of Microelectromechanical Systems, Vol. 2, No. 1, March 1993. This article is incorporated by reference herein. Directed pin-point actuation has been achieved using solenoid coils and micro-machined nickel-iron cores on the order of 25 microns wide and requiring a current of 800 mA for actuation.
Magnetic micro-platforms, on the order of one mm2, powered by local electromagnets, require a current of 182 mA for actuation due to improvements in the local magnetic source by reducing reluctance and using thinner micro-platforms which reduce the length the electromagnetic field must travel through air. Chang, Carl, et al., Magnetically actuated microplatform scanners for intravascular ultrasound imaging, MEMS-Vol. 2 Micro-Electro-Mechanical Systems (MEMS)xe2x80x942000, ASME 2000. This article is incorporated by reference herein.
Piezoelectric micro motors have been designed with diameters of 2-5 mm which require four volts at 90 kHz to generate 100-300 rpm. Flynn, Anita {overscore (M)}., Piezoelectric Ultrasonic Micromotors, Massachusetts Institute of Technology, PhD dissertation June 1995. This paper is incorporated by reference herein. These devices are ultrasonic and provide the advantage of a holding torque when the sound wave is not traveling between the stationary and rotating aspects of the motor.
Methods for fabricating such devices using processes similar to integrated circuit manufacturing have been suggested. Zettler, Thomas, Integrated circuit fabrication compatible three-mask tungsten process for micromotor and gears, Sensors and Actuators 44, (1994), pp. 159-163. This reference in incorporated by reference herein.
The problem with existing microdevices is that several units are necessary to pump, valve, mix, and meter blood and reagents. This is prohibitive where space is limited, such as in a hand-held point-of-care device for analyzing blood samples. For such an application compact design and mass manufacturing are necessary due to the size and biohazard constraints. Henceforth, the term xe2x80x9cbiological fluidxe2x80x9d will be used to mean bodily fluid samples, such as blood, and/or other reagent chemicals; such reagents preferably support a variety of analytical methods including electrochemical, chemiluminescence, optical, electrical, mechanical, and others, for determination of blood pH, pO2, pCO2, Na+, Ca++, K+, hematocrit, glucose, and coagulation and hemoglobin factors.
It is accordingly a primary object of the invention to integrate valving, pumping, mixing, and metering of biological fluid by incorporating the valve and pump mechanisms as an integral micro-pumping unit that can be manufactured at low cost such that the user can discard a device using such micro-pumps after a single use.
This is achieved by designing the micro-pump so that it is easily fabricated with existing MEMS and plastics technologies. The micro-pump is assembled within a disposable cartridge that operates in conjunction with a point-of-care analytical device. During the fabrication and assembly process of such a cartridge, the micro-pumps may be discretely fabricated and tested then assembled into the cartridge. Alternatively, the micro-pumps may be assembled within such a cartridge and tested on the actual cartridge itself once it has been inserted into the point-of-care analytical device.
In accordance with the invention, a microscopic paddle wheel is coupled inductively by an external electromagnet and is used for valving and active pumping. Such a system takes advantage that the actual pumping mechanism is completely isolated from the electromagnet driver. The paddle wheel is inexpensive to manufacture and disposable. A cartridge may have a network of conduits and reservoirs containing several such paddle wheels to transport biological fluid. A point-of-care device housing the electromagnetic driving mechanism is reused with successive cartridges since the paddle wheels are contained by the cartridge and do not contaminate the driving mechanism.
An inductive drive such as an electromagnet and a magnetic core incorporated within the paddle wheel to actuate the motion may be separated by either plastic or silicon approximately one millimeter thick and still maintain an inductive coupling with the paddle wheel such that the magnetic core spins by rotating the magnetic field. The electromagnet may be a micro-coil which causes the paddle wheel to move according to the alternating field in the micro-coil.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawing, which is incorporated in and constitutes a part of this specification, illustrates an embodiment of the invention and, together with the description, serves to explain the principles of the invention.